The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Corrosion can occur at a junction between devices formed of dissimilar metals due to galvanic action. In general, at a junction between dissimilar metals, the metal with a more negative potential corrodes preferentially. By way of example, when a device formed of magnesium is in physical contact with a device formed of aluminum in the presence of a corroding environment, e.g., an electrolyte such as salt, the magnesium device corrodes near the junction. This is known as galvanic corrosion. Galvanic corrosion occurs because the corrosion electric potential of magnesium is about −1.6 volts while that of aluminum is about 0.8 volts. Hence, the magnesium device becomes an anode, the aluminum device becomes a cathode, and there is a current exchange including dissolution of the metal on the anode (magnesium) side.
A known way to protect such type of galvanic corrosion in metals is to provide electrical insulation between the two devices. But insulating materials like gaskets are not readily employable in certain applications, e.g., an automotive engine cradle subject to high temperatures and adverse loading conditions.
A liquid galvanic coating for protection of embedded metals has been proposed, wherein a fluid galvanic coating for protecting corrosion-susceptible materials is embedded within a substrate. The coating includes one or more metals selected from the group consisting of magnesium, zinc and alumina or more elements and/or one or more additives selected from the group consisting of conductive polymers, carbon fibers and graphite.
Another proposed manner of protecting embedded corrosion-susceptible materials requires coating of an overall structure with a conductive paint and applying current by the use of an external power supply. Such systems are costly to install, require a continuous power supply and must be periodically monitored and maintained throughout the life of the structure. Sacrificial cathodic protection methods typically require the application of metallic zinc.
Another proposed method includes applying a coating that acts as an electrolytic barrier and a cathodic corrosion prevention system, applicable to ferrous and non-ferrous metal substrates. The method provides cathodic protection from corrosion by coating with polymers and sacrificial anodic metal particles. This coating system is formed by a process that includes premixing of an inherently conductive polymer with anodic metal particles to form an inherently conductive polymer/metal particle complex.
Another proposed method to protect metals from corrosion uses a type of coating called barrier coating. Barrier coatings function to separate metal from the surrounding environment. Some examples of barrier coating include paint and nickel and chromium plating.
Another type of coating used to protect metal is called sacrificial coating. The metal is coated with a material that reacts with the environment and is consumed in preference to the substrate it protects. These coatings may be further subdivided into chemically reactive, e.g., chromate coatings and electrochemically active or galvanically active, e.g., aluminum, cadmium, magnesium and zinc. The galvanically active coatings must be conductive and are commonly called cathodic protection.
There can be difficulty in creating a coating that protects like a cathodic system but is applied with the ease of a typical barrier coating system. Furthermore, there are environmental considerations related to plating operations and surface preparation for certain top coating processes.
Metallic spacers have been used in automobiles. In one example, an aluminum spacer has been placed between magnesium and steel, creating a junction consisting of magnesium-aluminum-steel. This junction generates electrochemical corrosion potentials of Mg: −1.6V/Al: −0.8V/Fe: −0.2V. The electrochemical corrosion potentials suggest that if the corrosion potential of a spacer has an intermediate value of the other components of the junction, then the galvanic corrosion is reduced. It has been found that a single alloy spacer is not substantially effective in preventing galvanic corrosion, particularly in automotive components formed of magnesium and exposed to high temperature operating environments.